Removal of carbon dioxide from air

ABSTRACT

The present invention is directed to methods for removing CO 2  from air, which comprises exposing sorbent covered surfaces to the air. The invention also provides for an apparatus for exposing air to a CO 2  sorbent. In another aspect, the invention provides a method and apparatus for separating carbon dioxide (CO 2 ) bound in a sorbent.

The present invention relates to removal of selected gases from air. Theinvention has particular utility for the extraction of carbon dioxide(CO₂) from air and will be described in connection with such utilities,although other utilities are contemplated.

Extracting carbon dioxide (CO₂) from ambient air would make it possibleto use carbon-based fuels and deal with the associated greenhouse gasemissions after the fact. Since CO₂ is neither poisonous nor harmful inparts per million quantities but creates environmental problems simplyby accumulating in the atmosphere, it is desirable to remove CO₂ fromair in order to compensate for emissions elsewhere and at differenttimes. The overall scheme of air capture is well known.

The production of CO₂ occurs in a variety of industrial applicationssuch as the generation of electricity power plants from coal and in theuse of hydrocarbons that are typically the main components of fuels thatare combusted in combustion devices, such as engines. Exhaust gasdischarged from such combustion devices contains CO₂ gas, which atpresent is simply released to the atmosphere. However, as greenhouse gasconcerns mount, CO₂ emissions from all sources will have to becurtailed. For mobile sources the best option is likely to be thecollection of CO₂ directly from the air rather than from the mobilecombustion device in a car or an airplane. The advantage of removing CO₂from air is that it eliminates the need for storing CO₂ on the mobiledevice.

Various methods and apparatus have been developed for removing CO₂ fromair. In one of these, air is washed with an alkaline solution or sorbentin tanks filled with what are referred to as Raschig rings. For theelimination of small amounts of CO₂, gel absorbers also have been used.Although these methods are efficient in removing CO₂, they have aserious disadvantage in that for them to efficiently remove carbondioxide from the air, the air must be driven by the sorbent at a fairlyhigh pressure, because relatively high pressure losses occur during thewashing process. Furthermore, in order to obtain the increased pressure,compressing means of some nature are required and these means use up acertain amount of energy. This additional energy used in compressing theair can have a particularly unfavorable effect with regard to theoverall carbon dioxide balance of the process, as tie energy requiredfor increasing the air pressure would produce its own CO₂ that wouldhave to be captured and disposed of.

Thus, since the prior art methods result in the inefficient capture ofCO₂ from air because these processes heat or cool the air, or change thepressure of the air by substantial amounts, i.e., the net reduction inCO₂ is negligible as the cleaning process introduces CO₂ into theatmosphere as a byproduct of the generation of electricity used to powerthe process.

Furthermore, while scrubber designs for separating CO₂ from air alreadyexist, generally they are limited to packed bed type implementationswhose goal typically is to remove all traces of an impurity from anothergas. One such device, described in U.S. Pat. No. 4,047,894, containsabsorption elements comprising porous sintered plates made ofpolyvinylchloride (PVC) or carbon foam assembled spaced from one anotherin a housing. Prior to the plates being assembled in the housing,potassium hydroxide is impregnated in the porous plates. Such a devicehas the disadvantage that the sorbent material used to separate CO₂ fromair cannot be replenished without disassembling the device housing.

Processes that collect CO₂ from the air typically rely on solvents thateither physically or chemically bind CO₂ from the air. A class ofpractical CO₂ solvents include strongly alkaline hydroxide solutionssuch as, for example, sodium and potassium hydroxide. Hydroxidesolutions in excess of 0.1 molarity can readily remove CO₂ from airwhere it becomes bound, e.g., as a carbonate. Higher hydroxideconcentrations are desirable and an efficient air contactor will usehydroxide solutions in excess of 1 molar. Sodium hydroxide is aparticular convenient choice, but other solvents such as organic aminesmay be used. Yet another choice of sorbents include weaker alkalinebrines such as sodium or potassium carbonate brines.

See also, PCT Published Applications PCT/US2005/015453 andPCT/US2005/015454.

The foregoing discussion of the prior art derives primarily from ourearlier Published PCT Application PCT/US05/29979 in which there isproposed a system for removing carbon dioxide from air, which comprisesexposing solvent covered surfaces to airstreams where the airflow iskept laminar, or close to the laminar region. The carbon dioxide gas isabsorbed by the solvent and removed from the air. In a preferredembodiment, the solvent comprises an alkaline sorbent solution such as astrong hydroxide solution. See also, our earlier published PCTApplication Ser. No. PCT(US06/03646 in which we describe an air/liquidexchanger comprising an open-cell foam for supporting a liquid sorbent.

The present invention provides improvements over the prior art asdescribed above. More particularly, the present invention providesseveral processes and systems for removing carbon dioxide or other gasesof interest from air.

In accordance with one embodiment of the invention, there is provided anion exchange material to capture or absorb CO₂. In one aspect, theinvention employs a solid anionic exchange membrane as the primary CO₂capture matrix. The ion exchange material may comprise a solid matrixformed of or coated with an ion exchange material. Alternatively, thematerial may comprise a cellulose based matrix coated with an ionexchange material.

Yet another embodiment of the invention employs a wetted foam airexchanger that uses a sodium or potassium carbonate solution, or otherweak carbon dioxide sorbent, to absorb carbon dioxide from the air toform a sodium or potassium bicarbonate. The resulting sodium orpotassium bicarbonate is then treated to refresh the carbonate sorbentwhich may be recovered and disposed of while the sorbent is recycled.

In yet another embodiment of the invention, carbon dioxide is removedfrom the air using an ion exchange material which is regenerated using aliquid amine solution which is then recovered by passing the aminesolution into an electrodialysis cell.

In still yet another aspect of the invention, carbon dioxide is removedfrom the air by modifying the alkalinity of seawater which in turnincreases the flux of carbon dioxide from the atmosphere into the water.

Further features and advantages of the present invention will be seenfrom the following detailed description, taken in conjunction with theaccompanying drawings, wherein like numerals depict like parts, andwherein

FIG. 1( a) is a side elevational view, in partial cross-section and FIG.1( b) is a perspective view of yet alternative forms of air scrubbersmade in accordance with another embodiment of the present invention;

FIGS. 2( a)-2(c), 3(a)-3(b) and 4(a)-4(c) are perspective or sideelevational views, as the case may be, of air scrubbing units made inaccordance with yet other embodiments of the present invention;

FIG. 5 is a block flow diagram illustrating a process for removing CO₂from air in accordance with one embodiment of the invention;

FIGS. 6-8 graphically illustrate the CO₂ capture over time;

FIG. 9 is a process flow diagram in accordance with an embodiment of theinvention;

FIG. 10 is a schematic flow diagram showing an integrated system for CO₂removal from air in accordance with another aspect of the invention; and

FIGS. 11-14 are schematic diagrams of cells for treating seawater inaccordance with alternative aspects of the invention.

The present invention generally relates to carbon dioxide (CO₂)extraction, reduction, capture, disposal, sequestration or storage,particularly from air, and involves new processes and apparatuses toreduce or eliminate CO₂ from the environment. Both extraction andsequestration of CO₂ are encompassed by the invention.

In our earlier U.S. Patent Application Ser. No. 60/603,811, we outlineda strategy for contacting air with sorbent coated surfaces. We showed,that with the slow reaction kinetics typical of hydroxide or carbonatesolutions absorbing CO₂, one should provide straight channels forlaminar flow to maximize the uptake of CO₂ for a given energy investmentin pressure drop across the collecting structure. If the liquid sidereaction kinetics could be improved, more complex channels would reducethe air side limitation, but for low reaction kinetics straight channelswith smooth surfaces appear most effective.

This invention in one aspect provides an approach for absorbing carbondioxide from an air stream that can proceed efficiently even with weaksorbents and at low uptake rates. By wetting a foam, which has straightchannels cut through it, in a manner that internal foam surfaces arefully or partially covered with a weak sorbent, it is possible to createa large area of sorbent surface that is exposed to a slow gas flow. Thegas flow through the channels and through the bulk foam can be adjustedso as to optimize the uptake of dilute carbon dioxide for a givenpressure drop across multiple layers of foam. For the extraction of lowconcentration gas admixtures to a gas stream this technique obviates theneed for strong sorbents with a fast rate of absorption. As aconsequence one can take advantage of weak sorbents like sodiumcarbonate for capturing CO₂ from air, rather than having to rely onstrong sorbents like sodium hydroxide. The lower binding energy ofcarbon dioxide to the weak sorbent greatly simplifies subsequent sorbentrecovery steps. This disclosure describes the principles involved andoutlines a method and apparatus to create moist foam surfaces and toextract the CO₂ laden sorbent from the foam. These methods can be usedwith any applicable sorbent recovery method. They are not limited to thecapture of carbon dioxide from the air, but could be extended easily tothe capture of trace gas admixtures from any gas stream. As outlinedbelow, the details of the implementation will depend on theconcentration of the trace gas, the rate of the adsorption or absorptionreaction and the flow speeds involved. It also matters whether the goalof the process is to capture all of the trace gas out of the mixture inorder to clean up the gas, or whether the goal is to collect a valuablestream of trace gas from the mixture without attempting to eliminatenearly all traces from the carrier gas.

In collecting carbon dioxide from the air, two distinct transfer stepscould potentially set the rate limit. The first is the uptake of carbondioxide into the sorbent, the second is the transport of carbon dioxidethrough an airside boundary layer to the surface of the sorbent. In thefirst case the capture system is sorbent-side limited in the second itis air-side limited. In an earlier published PCT Application Ser. No.PCT/US06/03646, we outlined one approach to optimizing a CO₂ capturedevice from a dilute stream. Here we outline another approach that takesadvantage of a very different principle. Both approaches aim to minimizethe pressure drop required across a scrubbing device for removing acertain fraction of the CO₂ from the air flow. Since CO2 in the air isvery dilute, it is important to minimize the energy penalty for pushingair through the air scrubbing system. Ideally, the pressure drop is sosmall, that the partial stagnation of natural wind flows is sufficientto provide the energy for making contact between the air and the sorbentmaterial.

The aforesaid previous invention provides a method of minimizing thepressure drop for a fixed flow velocity, by assuring that the CO₂transport is at least partially airside limited. For weak sorbents likealkaline solutions this suggests a laminar flow which generates boundarylayers thick enough to roughly equalize the air-side mass transportcoefficient and the sorbent side transfer coefficient. This invention bycontrast is concerned with partitioning the air flow into fast movingand slowly moving streams and inserting the scrubber into the streamwhere it flows slowly.

As a particular design we consider a filter device in which thedistances between nearest neighbor absorbing surfaces are small comparedto the allowable boundary layer thickness. In that case the CO₂concentration on the surface is not much reduced and consequently thesystem can be considered sorbent side limited. In such a system thefractional loss of momentum is large compared to the fractional loss ofcarbon dioxide. As one lowers the speed of the airflow, the systemremains sorbent-side limited and the fractional loss of momentum stillremains high, but the available momentum drops rapidly. Hence the totalloss in momentum is reduced for a given thickness of the filter system.The pressure drop can be even further reduced, as the longer residencetime of the air in the filter will lead to a higher reduction in CO₂content of the air. If one holds the fractional CO₂ extraction constantthe filter can be made thinner and thus the required pressure drop iseven further reduced.

However, if the total flow through the collector is to remain constantthe slowdown of the flow in the filter must be accompanied by a speed-upof another stream. This can be accomplished by partitioning an airstream into two streams. Both streams simultaneously are run through afilter. The system experiences a pressure drop which is governed by thethickness of the filter, and the flow speed of the air. In panel B) theflow pattern has been reorganized so that one stream is first expandedout, while the other part is made to converge. As a result the air inthe widening section slows down, while the air in the narrowing sectionspeeds up. At the point of maximum cross section a filter is installedinto the slow flow. Downstream from this point, the expanded air flow ismade to converge again and the other air stream fans out to the sameextended cross section the first flow had higher upstream. At this pointthe air in the second stream is scrubbed of all or part of its CO₂. Afinal section follows where both streams are readjusted to their initialcross-section. In order to achieve the same filter affiance, the filtersin this new design can be substantially thinner. If the system issorbent side limited, then the volume of the filter does not need tochange, but since the cross section increased the thickness of thefilter can be reduced accordingly. The pressure drop is reduced becausethe flow speed is lower and the resistance of the filter is reduced.

The above example serves to explain a basic physical principle. In thefollowing we outline a specific method of approximating such a behaviorwith simple blocks of foam like filter material. Foam blocks have manyadvantages: They can be shaped into arbitrary forms, they can hold someliquid and they are easily wetted; and open cell foams present a largeinternal surface area that can be used to absorb CO₂ from air flowingthrough the foam.

A large foam block wetted with a liquid sorbent like NaOH or Na₂CO₃ willabsorb CO₂ from the air. If we assume a typical pore size of about 1 mmand a specific area of about 4000 m⁻¹, then a typical uptake rate for asorbent surface of about 2 μmol m⁻²s⁻¹ would provide an uptake capacityof 8 mmol s⁻¹ for a foam block of one cubic meter. If we intend toextract 5 mmol/m³ from the air stream, the thickness of the apparatus atflow speeds of 3 m/sec would be about 2 m. However, the pressure drop ofsolid block of foam would be far too large to maintain such a high flowspeed. If, however, one opens up channels through the foam that let 90%of the air bypass a foam layer, and then mix the air again and gothrough another layer with 90% bypass, then the effective flow speed inthe foam is ten times smaller, the pressure drop is reduced by a factorof ten, and the uptake rate is virtually unchanged as it is not limitedby the rate at which air flows through the thin slices, but by the rateat which the surfaces inside these foam slices can absorb CO2.

By forming small straight channels through a layer of foam, one opens apathway through the foam that will allow the bulk of the air a path thatavoids going through the foam. By adjusting the total cross section ofthe holes, and the diameter of the holes it is possible to control therelationship between pressure drop and flow speed, and the fraction ofthe flow that actually goes through the holes.

Small diameter holes at a fixed flow rate will lead to a higher pressuredrop, or alternatively at a fixed pressure drop they will lead to ahigher flow rate. A practical system operates between the two limitswhere adjustment of the hole diameter and the number of holes willchange the overall resistance to flow and thus change pressure drop andflow speed.

Increasing the number of holes will increase the flow rate, and hencereduce the pressure drop across the foam block. The pressure drop acrossthe foam block in turn controls the flow speed through the bulk of thefoam. It is therefore possible to adjust the parameters of this systemin a way that optimizes a specific apparatus in that one controls itspressure drop, across the foam block, and independently the size of thebypass flow.

Finally, one generalization of these concepts: The concepts are notlimited to extracting CO₂ from air, but they can be easily generalizedto the extraction of any trace gas from any gas stream. Finally, whilein most of the above discussion we assumed that the absorber is a liquidthat is absorbed by the foam, it is of course also possible to considerfoam like solid materials, including mats of fibers or other structures,that can absorb CO₂ as it passes through the system.

In contrast to experiments performed with AQUAFOAM®, which is a veryhydrophilic phenolic foam that easily retains liquid and thus has porescompletely filled with liquid, the polyurethane foams were essentiallystripped of 80 to 90% of the volume of liquid it contained at the pointof immersion. In contrast to the experiments on phenolic foams(AQUAFOAM®), in experiments with polyurethane foam the duration ofuptake was greatly reduced from days or weeks to tens of minutes. Inreturn the rate of uptake was greatly enhanced for a weak sorbent like ahalf molar sodium carbonate solution. The critical difference betweenthe two experiments is that in the former experiments the foam is filledwith fluid, whereas in the latter the foam volume is in its majorityfilled with gas. Intermittent soaking of the polyurethane foam blockduring the experiment, which would fill the pore space with liquid, leadto an immediate reduction in CO₂ uptake which only recovered after theliquid level contained in the foam had been appropriately reduced.

While the CO₂ uptake of a carbonate solution is greatly enhanced, therate of water evaporation is essentially unchanged. Water evaporation isnot sorbent side limited and hence the gas stream moving through thefoam block is immediately water saturated and thus stops soaking upadditional water. However, in most designs it will not be possible totake advantage of this effect, as a system that maximizes CO₂ uptakewill contact all of the air and thus saturate all of the air with watervapor.

The role of hydrophilic vs. hydrophobic vs. mixed surfaces is at thispoint not fully understood. Each have advantages and disadvantages.Hydrophobicity controls the amount of liquid retained in the foam andthe ease with which this liquid can be applied evenly. Thus, it isbelieved that a hydrophilic phenolic foam with slightly larger thanusual pore sizes could combine excellent wetting properties with anappropriate low water retention level. Most commercially availablephenolic foams are designed to retain water, and thus are not wellsuited to this application.

Various foams are commercially available and can be used. These includehard foams that would crush and be mechanically destroyed if subjectedto significant compression, soft elastic foams that can be “squeezed.”Hard foams can only be flushed with fluid. In order to maintain anappropriate level of saturation, it is necessary to let such foamsdrain. On the other hand, it is possible to push liquid out of the foamby driving a gas like air under pressure into the foam matrix.

Unevenness in flow patterns, draining and drying rates can render theuse of these foams very challenging. In the case of soft, elastic foamsit is possible to move liquid into and out of the foam by compressingthe foam matrix. In the case of hard foams turning the foams will helpin evenly distributing fluid throughout the volume of the foam.

A second aspect of this first embodiment thus is concerned with theapplication and extraction of liquid from soft and elastic foamstructures as well as from foams that cannot be compressed withoutdamaging the foam structure.

The simplest approach to wetting the foam would be the application ofliquid on the top and letting it drain by gravity. Particularly largecelled foams, or reticulated foams which drain easily are suitable forthis approach. If wetting a foam is accomplished through flowing fluidsand gravity based drainage, then slowly rotating the foam aids inobtaining even fluid coverage inside the foam. The direction of the axisof rotation must have a component in the horizontal direction, so thatrotation does change the flow direction inside the foam as it changesthe alignment of the foam with the direction of gravity. Rotation speedsare matched to the foam and fluid flow properties such that the bulk ofthe fluid but not all in the time of a rotation can flow to the bottomof the foam volume. By shaping the foam appropriately it is evenpossible to transfer fluid in the process of rotating the foam piece. Asan example, the foam may be formed into a closed spiral shape 200 asdepicted in FIG. 1( a), and slowly rotated about its axis with its rimor periphery 202 dipping into a pan or sump 204 containing liquidsorbent fluid 206. Channels 208 may be formed through the foam to allowpassage of air. Alternatively, the foam may be formed into an openspiral shape 210 as depicted in FIG. 1( b) and slowly rotated with itsperiphery into a pan or sump 214 containing liquid sorbent fluid. Also,if desired, the central axis end of the foam spiral may be mounted in asorbent collection tray 216 which rotates with the foam spiral. Therotation in this case will gradually move the fluid from the rim of theshape to its center where it may be extracted from the foam.

In foams that can be elastically compressed, it is possible to assurefluid mixing by moving the fluid by compressing and relaxing the foam.Referring to FIGS. 2( a)-2(c), in order to move liquid through the foamstructure external pressure may be applied by moving rollers 42 over thesurface of the foam 44 or by compressing foam blocks between flatplates. Rollers 42 may be smooth cylindrical surfaces that roll on bothsides of the foam. The rollers push the external foam surfaces towardeach other and thus force fluid to flow and mix throughout the volume.Alternatively, one can use a single roller on one side, and a rigidsurface on the back of the foam holding the foam in place. Thisarrangement would be particularly useful for relatively thin foams,where the additional cost of a second roller and the concomitantstructural complications would not be justified.

Instead of having smooth surfaces the surfaces of the rollers can bestructured and shaped so as to increase the fluid movement in the foamby varying the degree of compression locally. Options include, simplefluting with ridges that follow the roller axis. Alternatively one canconsider ridges that run circumferential around the rollers, or surfaceswith dimples and protrusions. With any of these structured surfaces, itwould be useful to match the surfaces on the opposing rollers (or shapesin the structured walls) so as to optimize fluid flow patterns.Attention must be paid to maximizing volume change in the foam whileminimizing shear strain in the foam.

Referring to FIGS. 2( a)-2(c) particular implementation which we discusshere for illustrative purposes would be a foam matrix 44 rectangular inshape, with large width and height and a relatively small thickness, asan example consider a foam block collector pad, 2 meters high, 1 meterwide, and 0.3 m in thickness. Narrow tubular channels cross through theblock in the 0.3 m thickness of the block. Air would flow through thefoam in the direction of the channels, traversing the foam in thedirection of its smallest dimension. Liquid could be applied to its twosides or to the top, and rollers 112, 114 would span the rectangularfaces 2 m tall and 1 meter wide. The rolling action would squeeze liquidin place, a downward stroke with a high degree of compression could beused to squeeze liquid downward and let it drain from the bottom of theblock.

Rollers 112, 114 would move up and down the sides of the foam, and theymight move in or out to modify the compression on the foam collectorpad. An upward stroke with less compression could be used to establish auniform fluid filling throughout the brick.

Liquid 116 could be applied on the top of the brick and pushed down bythe rollers. Some fluid will be pushed downward, and depending on thegap between the rollers a certain amount of fluid is left behind in thefoam matrix. If the height of the foam is not too large all fluid couldbe applied on the top and pushed down to the bottom. Alternatively, wecan spray the fluid onto the sides of the foam in advance of therollers. If the compression is set high the rollers can be used tosqueeze out liquid hat is either captured directly in front of therollers as it pushes out of the sides of the rollers or if the speed ofthe rollers is sufficiently slow, the fluid will be pushed to the bottomof the foam pad, where it will be extruded and collected. It is thuspossible to remove liquid from the pad by either injection additionalcarrier fluid, or just squeezing out liquid from the foam. In a secondapplication fresh fluid is applied to the foam, which with a lower levelof compression is evenly applied over the volume of the foam pad.

It also is possible to move the pads through the rollers and install therollers in a fixed position.

Referring to FIGS. 3( a)-3(b), an alternative to rollers would be flatplates 118-120 squeezing the entire area of the foam collector pads 110.This would work particularly well for arrangements in which the airflowis aligned in the vertical direction and the compression of the foam isused to squeeze fluid in and out of the foam parallel to the air flowdirection, which usually represents the smallest dimension of the foampad. It is also possible to turn the foam pad prior to squeezing andmove it from an upright position into a horizontal position.

A particular implementation where the foam is moving rather than therollers would be design where the foam moves as a continuous loop, likea belt over rollers that saturate and squeeze the foam, while the foammoves in an endless loop. These loops could be arranged in various ways,in particular it is possible to run the loop vertically up and down, orrun it horizontally.

In yet another aspect of the invention, illustrated in FIGS. 4( a)-4(c),the collector may comprises a plurality of foam collector pad 50, eachrotatably suspended from a support post 132 which support posts 132 arein turn horizontally movable between an operating and open position asshown in FIGS. 4( a)-4(b), respectively, and a closed position showed inFIG. 4( c) in which a liquid sorbent may be applied from a spray andexcess sorbent squeezed via end plates 48. The amount of liquid presentis chosen such that gas flow through the foam sees little impediment,the bulk of the pore volume is filled with gas, and gas filled porespaces are interconnected so as to make it possible to transfer CO₂ bydiffusion or other means from one pore to the next, until it getsabsorbed.

For air side limited flows, channels are ideally straight, but theeffective rate of migration of sorbate gas into the foam structure maybe enhanced by creating pressure fluctuations in the flow field.

While sodium hydroxide solution may be employed as the sorbent in theabove described apparatus, i.e. in accordance with the teachings of ouraforesaid published PCT Application Ser. No. PCT/US06/03646, inaccordance with one embodiment of our invention we may employ a wettedfoam air extractor system that uses a sodium or potassium carbonatesolution—or any other weak CO₂ sorbent, to absorb carbon dioxide fromthe air and in the process forms sodium or potassium bicarbonate; asorbent recovery step that refreshes the carbonate sorbent bypercolating the bicarbonate brine over a solid sorbent, which in apreferred implementation is an ion exchange resin; a resin recovery stepusing a liquid sorbent, which in a preferred implementation is a liquidamine solution, and a CO₂ release which is accomplished either bythermal swing, pressure swing or electrodialysis.

Following CO₂ from the air through the apparatus, can thus be describedas follows: the air comes in contact with a weak sorbent, like sodiumcarbonate, that by virtue of its distribution over a foam surface canachieve uptake rates that are so high that air side transport startslimiting the CO₂ uptake. Once the solution has taken up sufficientamounts of CO₂, it percolates over a solid sorbent, for example an aminebased ion exchange resin that removes bicarbonate from the solution andthus restores its alkalinity. The CO₂ is now attached to the resin andis removed from the resin in a subsequent step, by washing the resinwith a another liquid sorbent, preferably an amine solution from whichone can then in a final step recover the CO₂. Here the options are athermal swing, a pressure swing, or an electrodialysis process.

Referring to FIG. 5, the steps of the process are as follows: capture ofcarbon dioxide from air on a carbonate wetted foam at step 250. In thisprocess, a wetted foam structure is exposed to ambient air which flowsthrough the system at speeds ranging from 0.1 m/sec to 100 m/sec, thepreferable range being from 0.5 m/sec to 10 m/sec and an optimal rangeof from 0.5 to 4 m/sec. These foam structures are shaped or arrangedsuch as described above so that they have passageways through which airflows and comes in contact with the wetted foam surfaces. The wettedfoam surfaces absorb carbon dioxide. In such case the CO₂ laden sorbentcontains bicarbonate ions. The following steps of the process will haveto recover the carbonate from a very dilute bicarbonate stream that ismixed in with a possibly much larger concentration of CO₃ ^(═).

The ratio of carbonate to bicarbonate depends on the total carbonconcentration. In order to have sorbent liquid move into and out of thefoam, liquid is flushed out of the foam by one of several methodsdescribed elsewhere. The preferred method would be a design wheregravity drainage of the liquid by itself will remove the spent sorbent,or a water flush will mobilize the spent sorbent and collect it at thebottom of the device. For implementations in which the optimal capturedesign does not lend itself to gravity drainage, other methods thatutilize motion or compression of the foam are possible such as describedabove.

In any case, the resulting solution contains a dilute stream of sodiumbicarbonate. Given the low concentration, direct sorbent and CO₂recovery from this brine is usually not the most advantageous approach.As an alternative we provide a three stage approach where the lowconcentration bicarbonate is first concentrated by bringing the solutionin contact with an amine based ion exchange resin.

In the next step 252, ion exchange resins in contact with bicarbonatesolutions will absorb bicarbonate ions from the brine, and replace themwith hydroxide ions, which in turn are neutralized by reacting with asecond bicarbonate ion resulting in the formation of carbonate ions andwater. Resins could be of various types, but several suitable resins areavailable commercially. Preferred are resins functionalized with aminegroups. The important consideration is the binding energy of thebicarbonate (or carbonate) to the resin. It must be large enough totransfer CO₂ from the liquid to the resin, but weak enough to relinquishthe carbon dioxide in the subsequent processing step. Typical bindingenergy would range from 20 to 60 kJ/mole but wider ranges are possible.While for practicality, organic resins are preferred, other solidsorbents equally could be utilized to perform this transition. Oneparticularly preferred material is magnesium hydroxide, although othersolid materials that can be carbonated may be used, such as lithiumsilicates and lithium zirconates which are given as examples. Suchmaterials are capable of absorbing CO₂ and may be used as solid sorbentsin accordance with the present invention. Similarly, variouscommercially available ion exchange resins are capable of recovering thecarbonate brine, by raising the alkalinity back to that of the startingmaterial may be used in the practice of the present invention.

A particular implementation is a resin bed through which CO₂ ladensorbent is cycled. As the sorbent flows through the bed the resin isgradually saturated with carbon dioxide. If flows are kept relativelyslow, the absorption front will move gradually through the resin untilit breaks out at the far end of the bed, at which stage one wouldobserve a sudden increase in the concentration of bicarbonate in theeffluent and thus know when the resin has been spent. Once this pointhas been reached, the resin is due to be refreshed.

The partial pressure of carbon dioxide in the air is very low, around380 micro bar. As a result for most resins, this front will be ratherwide and ill defined, in that case it would be advantageous to break theresin bed into multiple beds, and use a nearly spent bed, to begin theremoval of carbon dioxide and thereby maximize saturation of the bed,use a one or more cascading beds to remove the bulk of the CO₂ from thesorbent and percolate the sorbent fluid finally through a last freshbed, to maximize extraction. By plumbing and valving stationary bedstogether, it is possible to cycle their logical position in the chain ofsorbent refreshing or in the resin recovery step. As a result, the stepsof the operation move gradually through a ring of tanks. For someresins, the binding energy of different sites varies, and in that caseit would be disadvantageous to push the resin to its limits. Instead insuch a case the resin would swing back and forth within a range ofbinding energies that are easily accessible.

The resin is recovered in a step 254 by washing it with a different CO₂sorbent, for example, an amine solution that binds carbon dioxidestrongly enough to recover it from the resin. This will lead to atransfer of the bicarbonate, carbamate or carbonate ion from the resinto the amine solution. The advantage of this last step is that he aminesolution can achieve far higher load factors, i.e., ratio of aminesolution-to-CO₂ weight than the resin itself. The improvement is evenlarger, when compared to the initial carbonate brine. Thus less energyis wasted in heating and cooling the sorbent, than if the heat recoverystep would be performed on the resin itself or if recovery wereattempted from the original weak sorbent.

The amine solution loaded with CO₂ is transformed in a thermal swing torelease carbon dioxide from the amine in a step 256. There are severaloptions available for this step, since amine solutions are used in othercarbon dioxide absorption systems. In one option, steam is used intransferring heat to the process. Preferably, heat for forming the steamwill be from carbon neutral energy sources such as solar energy, orabsent these sources, from the combustion of carbon based fuels withpure oxygen, thereby creating an additional stream of concentrated CO₂that reflects the energy demand of the CO₂ recycling process. Of course,other heat sources including geothermal heat sources, solar energy heatsources, as well as waste heat energy sources may be used.

The invention is susceptible to modification. For example, instead ofusing inactive foam with a liquid sorbent percolating through it, it ispossible to use a functionalized foam or resin without the use of acarbonate sorbent. In such case, the wetted foam would be used todirectly collect carbon dioxide from the air. In such case, the foamshould not be allowed to dry completely, but for some foams it may notbe necessary to inject liquid water, since a minimum amount of moisturein the air may be sufficient to have the amine react with carbon dioxideform the air. Once the foam is saturated with CO₂, a flush with asecondary CO₂ sorbent may be used to regenerate the resin. This could bea carbonate solution, but with a higher concentration of sodiumcarbonate than in the system discussed above. The resin wash could alsobe an amine wash, in which case the process becomes a streamlinedversion of the main process discussed above.

Alternatively, instead of using carbonate sorbents in the foam one coulduse amine solutions directly in the foam. That would eliminate thesecond and third step of the process. The result is a process that isstreamlined down to a single process step for capture followed by asingle process step for sorbent recovery and CO₂ release.

It also is possible to replace the thermal swing for CO₂ recovery withan electrodialysis process. Electrodialysis could follow severaldistinct approaches, as disclosed, for example in our published PCTApplication PCT/US06/03646. Electrodialysis could be applied to thebicarbonate solution generated in the first step, or alternatively, itcould be applied to the amine solution that is generated in the finalstep.

In yet another aspect of the invention, we utilize solid phase anionexchange materials (AEM) for the direct capture of CO₂ and other acidgases from air. The application of AEMs as discussed herein with regardsto its utility for low (ppm) absorption of CO₂ from air, but readily isusable for capturing other low concentration gases such as NOxabsorption, and SO₄, as well as concentrated CO₂ or other gas removal.

Two alternatives are possible.

One alternative is to use an intermediate solid substrate that is ableto be exposed to large volumes of air and collect CO₂ at lowconcentrations while acting as a solid with little or no vapor pressure.The solid substrate can be envisioned to act as a sort of net, storingthe CO₂ chemically until it is released into solution at a later time.Further the solid substrate is able to release the newly collected CO₂back into a solution that also regenerates the solid surface. Thesolution containing the captured CO₂ is regenerated in an energeticallyfeasible way. A volatile or high vapor pressure solution can be utilizedto collect the CO₂ from the substrate and can be regenerated at lowenergy penalty. This intermediate step allows us to cleave CO₂ attachedto a substrate without exposing the substrate to the open environment,preventing atmospheric contamination and loss.

The above process exchanges anions to and from a solid substrate. Herewe are utilizing the anion exchange partner fastened to a solidsubstrate participating in ion transfer. An example of this is thereaction of methylamine onto a styrene backbone via chloro-methylation(a common ion exchange resin used in deionized water systems). In thistype of systems a nitrogen group such as an amine is attached to apolymer back bone via a covalent bond. This covalent bond pins theammonia type molecule to the substrate while allowing it to dissociate(to form a cation and anion). If all four of the possible covalent bondsthat can be attached to the nitrogen are filled with carbon groups, thenitrogen is forced into an electron deficient state and acquires apermanent positive charge. The permanent charge on the ammonium ionturns it into a cation which must then be satisfied by the ionicattachment of a neighboring anion. This is a salt that is covalentlyattached to a solid polymer substrate.

The ability to create a solid surface that acts like a strong basesolution provides several features and advantages not limited to thefollowing:

-   1. The CO₂ net utilizes the anion exchange properties of the amine    salt while capitalizing on the zero vapor pressure of the solid    polymer backbone. Essentially the amine salt can be forced into a    hydroxide form (OH⁻) by replacing all of its anions via    concentration gradient leaving a surface of OH⁻ attached to the    solid. The attached OH⁻s are now readily available for reacting with    incoming CO₂. Since most of the techniques to capture CO₂ exploit    the reaction of the acid gas with a liquid base, or OH⁻ surface,    this method shares in the fast acid/base reaction kinetics.-   2. The elimination of a liquid film intermediate allows for large    increases in surface area as compared to current methods. In gas    liquid contactors the challenge is to spread the liquid in such a    way as to contact as much air as possible. This normally involves    spreading the liquid over a solid surface to increase its surface    while not inducing such a large pressure drop that the gas is not    able to properly flow. The solid OH⁻ surface allows for maximum    surface area with minimal pressure drop.-   3. Minimal water is required for the reaction to occur and overall,    essentially no water is consumed. The membrane is able to cleave    water from the air in order to facilitate the capture. When large    volumes of air are concerned this is a major benefit.-   4. Because the OH⁻ is attached to the polymer substrate, it is no    longer able to react with the environment unless there is an anion    available to replace it or an acid is available to react with it.    This is a benign surface that is highly reactive with acid gases    only. This allows the complete removal of a strong oxidizer from    direct contact with the environment while still facilitating    capture.-   5. Another problem with contacting large volumes of open air is    airborne contamination of the collector itself. The buildup of dirt    and bacteria within the system is inevitable. As long as there is no    anion transfer to the solid itself from the contaminants, the    surface can be washed with water before being treated or    regenerated, eliminating contaminates from entering the rest of the    separation process.-   6. Little or no liquid pumping is required between surface renewals.    This significantly reduces pumping costs from distributing the fluid    over a surface to create contact area.-   7. Since the process for attaching anion exchange groups to polymers    is relatively well understood, there is no limit to the types or    shapes of materials to which the anion exchange material could be    applied.

In one aspect our invention employs solid anionic exchange membranes asthe primary capture matrix for the capture and subsequent delivery ofatmospheric CO₂. The membranes are spaced closely together with spacingsfrom 1-25 mm. This spacing allows for the passage of ambient air with apressure drop sufficiently low to preclude the use of machines to movethe air. This is in accordance with the matrix construction discussedour aforesaid PCT Application Ser. No. PCT/US05/29979.

The advantages of using ion exchange membranes as the material for thematrix are several. One advantage lies in the fact that the membranescan be operated in such a way as to be nearly dry, thus removing therisk of spreading caustic materials through the environment in the formof aerosols. Another advantage in operating in an essentially dry modeis the absence of water loss due to evaporation. This water loss issignificant not only in the amounts of water lost to evaporation, butalso in all the attendant costs of pumping, purchasing and plumbing ofthe water delivery systems. Another advantage is the membrane's abilityto store the captured CO₂ at a concentration greater than that possiblewith an aqueous surface of the same area. The increased apparent activearea exceeds the equivalent aqueous area. This allows capture at ratesthat exceed those possible by using aqueous solutions. Additionally, thetotal capture capacity is in excess of that possible with aqueoussolutions.

The attached FIGS. 6-8 which illustrate the CO₂ capture performance ofan anionic membrane exposed to both a continuous ambient air flow andalso a closed container (18.9 L) within which a small piece (2×2 cm) ofactive membrane is suspended and the drawdown of the CO₂ in the enclosedbottle is measured and logged.

Another data set shows a small piece of active membrane suspended withina larger (128 L) closed container with same data measured and logged.

In yet another aspect, the present invention employs cellulose basedpads as substrates for ion exchange media (IEM). As noted supra, IEMworks by allowing ions to exchange from a solution with like charged ionwithin the IEM. This exchange can be accomplished via several routes.

In one process a high concentration fluid induces like charged ions onthe IEM to migrate away from the resins' ion receptive sites into thesolution and allow the higher concentration ions in the solution tooccupy the sites. This can be envisioned as overpowering the resin via aconcentration gradient.

The absorption of CO₂ on an IEM takes place via the following mechanism:CO₂+H₂O→H₂CO₃H₂CO₃→H⁺+HCO₃ ⁻Resin(OH⁻)+H⁺+HCO₃ ⁻→Resin(HCO₃ ⁻)

Cellulose based IEM's have become very efficient. Using the EDM methodof animolysis to functionalize cellulose into an IEM has shown almostequivalent storage attributes as the commercially available IEM that arebased on styrene divinylbenzene. This provides the pathway for celluloseutilization.

We have found that IEM's have the ability to capture CO₂ directly fromthe air and release it via concentration gradient into an amine washsolution. This has many implications

Due to the large regeneration energy requirements of carbonated earthalkaline solutions, the use of amine based alkaline solutions has showna significant energy advantage. The problem, however, is that most aminesolutions that exhibit the desirable qualities required, such as highkinetic rate and absorption capacity, also exhibit a high partialpressure. Due to the large amounts of air that must be contacted tofacilitate the absorption, (around 2 million cubic meters per ton of CO₂assuming 200 ppm uptake), even low vapor pressure solutions have a veryhigh loss rate. Without an intermediate between the liquid amine and theair most amine solutions would not be applicable to the direct captureof CO₂ from the air.

The IEM is just that intermediate, which allows us to minimize thecontact time of the absorbent solution with large volumes of air butstill take advantage of the low regeneration costs of the liquid aminesolution.

Since the surface of the contactor is produced from the sorbent itself,there is no need to constantly wet a surface with sorbent liquid tofacilitate absorption. This is possible because the IEM's retainsignificant amounts of water (some as high as 50% by mass). This coupledwith an internal concentration gradient allows the IEM to act as asolution. As CO₂ is absorbed onto the IEM a concentration gradient isinduced that causes the migration of HCO₃ ⁻ away from the surface to alower concentration and the counter migration of OH⁻ to replace it. Thiseffectively allows the IEM to store HCO₃ ⁻ deep within its structurewhile not losing effective surface area. Of course, once the IEM becomessaturated after a set amount of time, the amine wash solution could beused to regenerate the media to its OH⁻ state and lose very little aminein the process. Our experiments have shown absorption periods of greaterthan 8 hours.

By eliminating the use of a continuous free passing ionic liquid solventwe also eliminate the formation of crystals on the collector surfacewhich ultimately will lead not only to decreases in the collectorperformance but also in the lifetime of the substrate. The IEM willcircumvent this issue molecularly by storing the ionic products withinthe substrate itself. Instead of the salt residing on the surface of thesubstrate where it can form scale and cause fouling, the anions that areproduced in the CO₂ absorption process have no counter ions which willenable them to sit independently on the surface of the media When theCO₂ is absorbed to the surface of the media, it effectively neutralizesthe OH⁻ anion portion of the resin replacing it with an HCO₃ ⁻effectively storing it in the substrate.

Yet another embodiment of the invention is a process for regenerating anion exchange resin used in capturing CO₂. FIG. 9 exhibits the generalprocess flow diagram.

To achieve separation and recovery of Na₂CO₃, CO₂ is removed from theNaHCO₃ in passing the liquid through an ion exchange media, in which CO₂is released, which undergoes an acid/base reaction with the NaHCO₃remaining in the liquid, thus regenerating Na₂CO₃. The Na₂CO₃ solutionthen exits the ion exchange column and is returned to the upstreamprocess.

The ion exchange media will over time become saturated with CO₂ and mustbe regenerated. This is achieved by passing a liquid amine solutionthrough the bed after the Na₂CO₃+NaHCO₃ stream has been removed. Theliquid amine solution will release an OH⁻ to the ion exchange resin,which in turn releases the CO₂, effectively regenerating the ionexchange media. The amine-CO₂ solution is then removed and the processis repeated as a cyclic system.

The amine-CO₂ solution must also go through a recovery step in order tocomplete the cycle. The amine-CO₂ recovery is accomplished in adistillation in which the CO₂ is separated and captured in the gas phaseand the amine-OH solution is returned to the bed.

The following non-limiting example further illustrates this aspect ofthe invention. A strong base macro-reticulated ion exchange resin wasused to cleave HCO₃− from NaHCO₃ into the ion exchange resin byreleasing OH⁻ ions into solution, therein creating Na₂CO₃. The solutionthat had passed through the resin was then titrated to measure thequantity of Na₂CO₃ produced from the ion exchange. The resin was thenthoroughly washed until there was no Na₂CO₃ or Na₂CO₃ left in the resin.The washed resin was then divided into two equal parts by volume andeach part was contacted with a liquid amine solution, one was contactedwith a primary, the other with a tertiary amine. The primary amine (MEA)and the tertiary amine (MDEA) were each used to remove the CO₂ that wasstored in the resin. The MEA solution showed a greater ability to cleavethe carbonate from the resin, while the MDEA solution exhibited,similar, but slightly lower absorption ability. Each amine-CO₂ solutionwas then titrated to verify the presence of the CO₂ within the solution.

Yet another aspect of the invention is illustrated in FIG. 10 whichprovides an integrated system for CO₂ removal from ambient air on an ionexchange member (IEM) 502. The CO₂ removal from the air by an IEM iswashed from the IEM by sodium hydroxide delivered from a sodiumhydroxide supply tank 504, producing sodium carbonate (Na₂CO₃) solutionwhich is collected in collection tank 506. The sodium carbonate solutionis electrolyzed in an electrolytic cell 508 wherein sodium hydroxide isrecovered and returned to tank 504. A portion of the sodium carbonatesolution is also passed to the tank 510 in which the sodium carbonate ispassed to a reactor 512 wherein the sodium carbonate is reacted withacetic acid to produce sodium acetate which is passed to aelectrodialysis stack 516 which regenerates sodium hydroxide and aceticacid from the sodium acetate salt feed. The acetic acid is returned totank 514 where it is used for subsequent mixing/reaction with the sodiumcarbonate from tank 510, while the sodium hydroxide is returned to tank504. Oxygen and hydrogen are collected or vented at outlets 518, 520,while CO₂ is collected and disposed of, e.g. by deep well injection orother means at outlet 522.

Yet another aspect of the invention employs seawater, i.e. the ocean, asa collector for CO₂. The mixed layer of the ocean, roughly the tophundred meters, are in chemical delayed equilibrium with the atmosphereand carbon dioxide in the air readily exchanges with dissolved inorganiccarbon in this layer. The dissolved inorganic carbon is in equilibriumwith the partial pressure of CO₂ in the air. Carbon dioxide will enterthe water either if the carbon dioxide partial pressure in the airincreases or, alternatively, if the alkalinity of the ocean water isincreased. The concept of introducing alkalinity into seawater as amechanism for capturing CO₂ from air is described in PCT/US2005/015453.The present invention provides improvements over this concept.

The alkalinity of seawater can be modified by either adding a base tothe water or by removing an acid. In one case, alkalinity of seawatermay be increased by extracting hydrochloric acid from the water. Inanother case, alkalinity may be increased by introducing a base that isobtained by splitting a salt, usually but not always sea salt, into anacid an a base. The base is added to seawater in a very dilute form,while the acid usually in a more concentrated form is retained forfurther processing and/or recovered for industrial use.

In order to reestablish equilibrium with the atmosphere, the water willabsorb carbon dioxide from the air, until the CO₂ uptake hasquantitatively matched the change in alkalinity.

Ocean water will absorb approximately one mole of carbon dioxide fromthe air for every mole of one-normal acid formed. A slight mismatch isdue to the fact that inorganic dissolved carbon is not completelybicarbonate, but a small fraction that is present as carbonate ions.Thus the effective normality of carbonic acid in seawater is slightlyhigher than its molarity. Reestablishment of the carbonate equilibriumwill occur on a short time scale of less than one year and thus willhappen without human intervention, except in places where surface watersare rapidly sinking. Thus, nearly the entire ocean surface is suitablefor this form of CO₂ management. An advantage of this aspect of theinvention is that it obviates the need for air exchange apparatus toremove carbon dioxide from air. The actual act of carbon dioxide captureis performed spontaneously and without the need for a sorbent orphysical collector installations.

As a result of this process, one is left with an acid other thancarbonic acid, typically hydrochloric acid which is much stronger thancarbonic acid, and therefore can more readily be neutralized by mineralbased alkalinity. Thus, rather than trying to dispose of a weak acidlike carbonic acid which is difficult to bind with mineral base, wegenerate a much stronger acid which is more readily neutralized byreadily available minerals that have a low level of reactivity.Alternatively, the hydrochloric acid may be collected for industrialuse.

The capture of carbon dioxide from the atmosphere by removinghydrochloric acid from ocean water could occur along the coast, or inthe middle of the ocean on board a ship. The important thing is that theacid extraction is performed on seawater that is in the mixed surfacelayer and which therefore will be exposed to carbon dioxide in the airwithin weeks or months after it has been processed. Rather than bringingcarbon dioxide to the disposal one removes hydrochloric acid from theocean water. An important advantage of this approach is that it requiresonly minute modifications in alkalinity of a local area of the ocean,whereas addition of carbon dioxide without changes in the alkalinitygreatly changes the carbonate chemistry of seawater. Furthermore, theCO₂ captured by excess alkalinity is stable and will not be releasedback into the air.

For such a sequestration method to become viable, it is necessary todispose of the large volumes of hydrochloric acid that will be producedin this process. One possibility is to dispose of the hydrochloric acidby neutralizing it with readily available alkaline minerals such asbasalt or serpentine rock. As an alternative, the hydrochloric acid canbe injected underground into alkaline fluid reservoirs that canneutralize the acid. Yet another possibility is to use mined and groundup minerals that can be transformed with the acid. These processes allare known in the art and have been published in the literature. Herethey are combined with a specific process for capturing carbon dioxidefrom the air to develop a method of carbon dioxide management that isdistinct from other approaches to the problem.

In the case where magnesium and calcium chlorides are formed, if theminerals are clean, they may be reinjected into the ocean where theyreadily dissolve. Alternatively, the resulting brines could be injectedunderground.

In yet another aspect of this invention, an electro-dialysis device isused to extract hydrochloric acid from seawater. The result is to createconcentrated hydrochloric acid which may be collected and usedindustrially, while barely changing the water chemistry of the oceanwater that is passed through the system. This is accomplished by flowinga large volume of seawater through the cells that collect the base,while running a small material flow through the cells that turn moreacidic.

The input on the basic side is seawater, which is converted to seawaterwith a very small change in alkalinity. Ideally the change is so smallthat the local water chemistry is not much affected by the change. Sincethe total alkalinity in seawater is about 2 millimolar, changes could bekept much smaller than that. In practice, it may be useful, to haveslightly larger changes and force dilution in the seawater stream at theexit of the system. On the other hand, in order to avoid fouling changesshould still be kept as small as possible. Fouling could easily occurwhen solubility products in the mixture are changed by significantfactors.

The input to the acidic side may be seawater or it could also be purewater, or any other brine that is available. Specifically, it ispossible to have multiple stages in the creation of hydrochloric acidand thus the input of that least some of the cells could be ahydrochloric acid solution that upon its exit has been strengthened inits molarity. FIG. 11 shows an example of such a device. This design isbased on a particular approach that eliminates the use of cationicmembranes which are usually present in an electrodialysis stack.

There are two substantively different approaches. The first has a numberof cells separated with anionic and bipolar membranes. The anionic andbipolar membranes alternate, with the stack completed on one end with ananode and at the other end with a cathode. Seawater in the larger cellswill receive hydroxide ions from the bipolar membrane and lose chlorideions through the anionic membrane. The acid forms in the complementarycompartments which receives protons from the bipolar membrane andchloride ions through the anionic membrane. Since the flow here is lowthe acid concentration will rise significantly, whereas the change inthe seawater chemistry is kept small.

FIG. 11 is a sketch of a repeated section of an electrodialysis devicefor extracting hydrochloric acid from ocean water. The oceancompartments experience high flow in order to minimize the chemicalchange in the water. In contrast the flow rate in the acidic cell isvery slow, so as to maximize the concentration of the resulting flowacid. It is possible to use the output of one HCl cell as inflow intothe next one. As a result the pH step is not everywhere maximized.

Flow rates on the alkaline and acidic side of the cells may differ byorders of magnitude. On the other hand, it is possible to achieve thesame effect by reusing the acidic fluid multiple times before it is asend out as a product stream. In either design, the output streams areslightly modified ocean water and concentrated hydrochloric acid. Withone mole of hydrochloric acid removed from the ocean water, the waterwill absorb from the atmosphere an amount of carbon dioxide thatrepresents one mole of CO₂. Since carbonic acid in seawaterdisassociates mainly into bicarbonate ions and protons, with a smallcontribution from carbonate ions, it requires approximately 1 mole ofCO₂ to compensate for the amount of hydrochloric acid withdrawn.

It is possible to build an electrodialytic device without cationicmembranes because the concentration of chloride ions in the ocean waterwill always far exceed the concentration of hydroxide ions, as the pH isbarely changed in the process. As a result the bipolar membraneseparates two fluids where on both sides the dominant negative ion is achloride ion. The complementary device which is build exclusively withcationic membranes alternating with bipolar membranes would not workwell. In this case HCl would be formed by transferring sodium ions outof the HCl cell through the cationic membrane. Once HCl has started toform protons would compete with sodium in the transfer and thus create alarge inefficiency.

It also is possible to design a conventional electrodialysis device withthree different membranes alternating in the design. In that case a saltis split into its anion and cation. The cation is added to an oceanwater flow, the anion ends up in the acid compartment. If the reductionin alkalinity in the acid compartment starts with highly alkaline brine,that is never neutralized, then it is possible to eliminate the anionicmembrane. In that case one in effect combines neutralization of the acidand production of the acid into a single step.

FIG. 12 is a sketch of an electrodialytic device that alternatescationic, anionic and bipolar membranes. The cells are arranged to raisethe alkalinity of seawater on one side of the bipolar membrane andcreate HCl on the other side of the bipolar membrane. The cell betweenthe cationic and anionic membrane contains a salt solution, in this caseseawater, which is diluted in its concentration.

A design shown in FIG. 12 allows the separation of seawater into aslightly diluted seawater stream, a slightly more alkaline stream andinto a stream of separated acid. A preferred implementation might usethe slightly diluted stream of seawater obtained from salt splitting andmake it the input stream for the seawater that will leave with increasedalkalinity. It is also possible to use a different salt in the cell fromwhich anions and cations are removed. A particular example would be theuse of a sodium salt of a weak acid; in that case the acid produced inthe last chamber would not be HCl but a different acid that again wouldbe ready for disposal. See FIG. 13.

FIG. 13 illustrates an electrodialysis cell stack that utilizes adifferent salt with anion X, to raise the alkalinity of seawater whilecreating a separate acid HX ready for disposal. For illustrativepurposes we assume that the salt is NaX, however, any cation that couldbe safely injected into ocean water could be used in the salt brine.

FIG. 14 illustrates a salt splitting cell without anionic membranes thatimmediately injects the produced acid into geological subsurface brine.The embodiment laid out in FIG. 14, combines electrodialysis with thedisposal of the hydrochloric acid, while avoiding the need for anionicmembranes. If the geological brine is more alkaline than seawater, it ispossible, with appropriate membranes to have the system run withoutinput of electricity, as it acts as a battery driven by the pHdifference between the ocean water and the brine. Thermodynamicsspontaneously will move toward reducing the pH difference between thetwo fluids. Since no cell operates at high acidity, it is possible toeliminate the anionic membrane which is used in the standard saltsplitter.

Various changes made be made in the above without departing from thespirit and scope of the invention as described. By way of example, theair capture exchange membrane may be in the form of elongate threads,typically 0.1-10 mm wide, preferably 1-10 mm wide, forming a loose matthrough which the air is flowed. The air capture exchange membrane alsomay be in the form of tubes, honeycomb structure or a foam structure. Itis intended that all subject matter contained in the above description,as shown in the accompanying drawings or defined in the following claimsto be interpreted as illustrative, and not in a limiting sense.

1. An apparatus for the capture of CO₂ from ambient air, the apparatuscomprising a store of ambient air, an anion exchange material in contactwith a flow of said ambient air, and a collector that collects CO₂separated from said anion exchange material subsequent to the capture ofsaid CO₂ by said anion exchange material.
 2. The apparatus of claim 1,characterized by one or more of the following features: a. wherein theanion exchange material is in the form of a plurality of membranesarranged in a series of flat plates with the ambient air flow parallelto face surfaces of the membranes; b. wherein the anion exchangematerial is in the form of a plurality of membranes spaced from oneanother with membrane spacings of from 1-25 mm, exposed to ambient airflow; c. wherein the anion exchange material is in the form of aplurality of membranes spaced from one another, and further comprising apowered blower for moving air across surfaces of the membranes; d.wherein the anion exchange material is in the form of one or moremembranes arranged in a corrugated fashion to allow air flow throughchannels formed by the corrugations; e. wherein a flow path along theanion exchange material has a length dictated by air flow velocity suchthat the ambient air is not excessively depleted of CO₂; f. wherein theanion exchange material is in the form of a plurality of membranesstacked in layers and forming “pie shaped” sections assembled into awheel, rotating on an axle; g. further including a caustic solution washfor washing captured CO₂ from a surface of the anion exchange material,wherein the wash preferably comprises an amine or a carbonate solution;h. wherein the anion exchange material is in the form of a continuoussheet which is moved in a serpentine pattern such that the membranefirst is exposed to air flow, dipped in a caustic, amine, or carbonatesolution, and then exposed to the ambient air repeatedly; and i. whereinthe anion exchange material is in the form of a loose mat of threads ortubes, or a honeycomb structure or a foams structure.
 3. A method forthe capture of CO₂ from ambient air, the method comprising: exposing ananion exchange material to a flow of ambient air; capturing CO₂ from theambient air with said anion exchange material; separating said CO₂ fromsaid anion exchange material; and subsequently collecting said separatedCO₂.
 4. The method of claim 3, characterized by one or more of thefollowing features: a. wherein the anion exchange material is in theform of a plurality of membranes arranged in a series of flat plates,and the air is flowed substantially parallel to surfaces of themembranes; b. wherein the anion exchange material is in the form of aplurality of membranes spaced from one another with membrane spacings offrom 1-25 mm, and air is flowed past the membranes without auxiliaryboost; c. wherein the anion exchange material is in the form of aplurality of membranes spaced from one another, and a blower is used topower air flow across surfaces of the membranes; d. wherein the anionexchange material is in the form of a plurality of membranes havingmembrane surfaces, and further wherein air flow and said membranesurfaces are matched so that the air is not excessively depleted of CO₂;e. wherein the anion exchange material is in the form of a plurality ofmembranes stacked in layers and forming “pie shaped” sections that areassembled into a wheel, and including the step of rotating the peripheryof the wheel through a CO₂ sorbent; f. including the step of washing theanion exchange material with a wash solution after exposure to the airso as to wash captured CO₂ from the surface of the anion exchangematerial, so that the anion exchange material can repeatedly act as aCO₂ absorber; g. including the step of washing the anion exchangematerial with a wash solution which comprises an amine solution, or acarbonate or bicarbonate solution; and h. wherein the anion exchangematerial is in the form of a continuous sheet, and including the stepsof moving the sheet in a serpentine pattern such that the sheet is firstexposed to air flow, dipped in a wash solution, and then exposed to theair repeatedly.
 5. A method for capturing CO₂ from ambient air, themethod comprising: exposing an anion exchange material to a flow ofambient air; capturing CO₂ from the ambient air; releasing said capturedCO₂ using a thermal swing, a pressure swing, or an electrodialysisprocess; and subsequently collecting said separated CO₂.
 6. A method forthe capture of CO₂ from ambient air, the method comprising: exposing ananion exchange material to a flow of ambient air; capturing CO₂ from theambient air with said anion exchange material; separating said CO₂ fromsaid anion exchange material; and subsequently disposing of said CO₂ bydeep well injection.
 7. The method of claim 5, wherein said captured CO₂is released using a thermal swing.
 8. The method of claim 5, whereinsaid captured CO₂ is released using an electrodialysis process.
 9. Themethod of claim 3, 5, or 6, further comprising storing said collectedCO₂.
 10. The method of claim 3, 5, or 6, whereby amount of CO₂ in saidambient air is reduced.
 11. The method of claim 3, wherein saidcollecting is preceded by a recovery step comprising a thermal swing, apressure swing, or an electrodialysis process.
 12. The method of claim3, 5, or 6, wherein said anion exchange material comprises an amine. 13.The method of claim 12, wherein said amine is covalently bound to fourcarbon groups.
 14. The method of claim 3, 5, or 6, wherein said anionexchange material is regenerated in a process having a low energypenalty.
 15. The method of claim 3, 5, or 6, wherein said anion exchangematerial comprises a plurality of membranes.
 16. The method of claim 15,wherein said plurality of membranes are arranged in series.
 17. Themethod of claim 15, wherein said plurality of membranes are spaced apartfrom one another with spacing from 1 mm to 25 mm.
 18. The method ofclaim 3, 5, or 6, wherein said anion exchange material has a thicknessfrom 0.1 to 10 mm.
 19. The method of claim 15, wherein said plurality ofmembranes are stacked in layers forming pie shaped sections that areassembled into a wheel.
 20. The method of claim 3, 5, or 6, wherein,said anion exchange material comprises tubes.
 21. The method of claim 3,5, or 6, wherein said anion exchange material comprises a foamstructure.
 22. The method of claim 3, 5, or 6, wherein said anionexchange material comprises a honeycomb structure.
 23. The method ofclaim 22, wherein said anion exchange material comprising a honeycombstructure comprises spacing from 1 mm to 25 mm.
 24. The method of claim22, wherein said honeycomb structure has a thickness from 0.1 mm to 10mm.
 25. The method of claim 3 or 5, further comprising disposing of saidseparated CO₂ by deep well injection.